1. Field of the Invention
The present invention relates to a method of wastewater treatment, and more particularly, to a method of fluoride-containing wastewater treatment.
2. Description of the Prior Art
Fluoride compounds have been broadly employed in many industries. In the dry etching process or the chemical vapor deposition process of semiconductor manufacturing, for example, a great amount of fluoric acid (HF) or ammonium fluoride (NH4F) is used. Consequently, a large quantity of fluoride-containing wastewater is generated. The fluoride-containing wastewater has to be processed for reducing the concentration of fluoride so as to prevent environmental pollution and to recycle fluoride.
Among various methods of fluoride-containing wastewater treatment, a calcium salt precipitation method is mostly used for its advantages of low cost, operation convenience, and short reaction time. The calcium salt precipitation method is implemented by adding a fixed amount of calcium salt, which reacts with fluoric ions in the wastewater, to form calcium fluoride precipitate as the following reaction shows.Ca2++2F−→CaF2
Then a solid-liquid separation process is performed to separate calcium fluoride from the wastewater so that the concentration of fluoride is reduced and the fluoride compound is recycled.
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a schematic diagram of a conventional fluoride-containing wastewater treatment system 10, and FIG. 2 is a flow chart illustrating the operation procedure of the fluoride-containing wastewater treatment system 10 shown in FIG. 1. As shown in FIG. 1 and FIG. 2, the fluoride-containing wastewater generated in the factory is temporarily reserved in a storage tank (not shown). When the wastewater is accumulated to a certain amount, the fluoride-containing wastewater is introduced to a neutralization tank 12 to perform a neutralization process 52. The neutralization tank 12 has a PH meter to measure PH value, so that the fluoride-containing wastewater can be maintained in a neutral condition by adding sodium hydroxide or hydrochloride.
The fluoride-containing wastewater is then delivered to a first reaction tank 14 to perform a primary calcium chloride addition process 54. When the primary calcium chloride addition process 54 is accomplished, the wastewater is then delivered to a second reaction tank 16 to perform a secondary calcium chloride addition process 56 so that the calcium ions of calcium chloride react with the fluoric ions of the wastewater and form calcium fluoride. The calcium fluoride and the wastewater are then delivered to a rapid mixing tank 18, a slow mixing tank 22, a settling tank 24, and a concentration tank 26 therein the calcium fluoride is separated from the wastewater by performing a solid-liquid separation process, such as adding agglutinators or flocculants. After that, the wastewater is able to meet the wastewater discharge standard, and a draining process 62 is performed to discharge the wastewater.
Generally speaking, the primary calcium chloride addition process 54 performed in the first reaction tank 14 is a preliminary reaction which works to remove approximately 70% to 90% of fluoric ions. Therefore, the dosage of calcium chloride is determined quantitatively by experimental data. In other words, the dosage of calcium chloride is fixed. On the other hand, the secondary calcium chloride addition process 56 performed in the second reaction tank 16 is dynamic. This means a feed back control 60 is used to dynamically adjust the dosage of calcium chloride in the secondary calcium chloride addition process 56 according to the result of a fluoric ion detection process 58 which detects the concentration of fluoric ions in the settling tank 24. For example, if the concentration of the fluoric ions detected in the settling tank 24 is excessive, the dosage of calcium chloride in the secondary calcium chloride addition process 56 is accordingly increased to reduce the concentration of the fluoric ions in the wastewater.
It is to be noted that the wastewater contains not only the fluoride, but also considerable sulfuric acid (H2SO4), nitric acid (HNO3), and phosphoric acid (H3PO4), therefore calcium sulfate (CaSO4), calcium nitrate (Ca(NO3)2), and calcium phosphate (Ca3(PO4)2) are also formed as well as calcium fluoride precipitate when calcium chloride is added into the wastewater. This leads to waste of calcium chloride. In addition, the concentration of fluoric ions is not stable. For example, if the concentration of fluoric ions changes dramatically, such as a transient high value appearing, the secondary calcium chloride addition process 56 will fail to control the concentration immediately. This makes the discharged wastewater have a high fluoric ion concentration, and thus causes environmental pollution.